Several studies concerning application of functional composite materials using a shape memory alloy have been hitherto proposed. It is known that concentration of a stress in an epoxy resin may be relaxed by embedding therein pre-strained shape memory alloy wires therein.
It is also known that when pre-strained shape memory alloy wires are embedded in a matrix of CFRP, GFRP, Al, etc., the vibration controlling function and fatigue crack developing rate can be retarded (Japanese Published Unexamined Patent Application No. H09-317821, H06-264161, H07-48673, H06-212018, H09-176330, etc.).
The above known technique utilizes an effect that an elongation strain imparted to the wires in a low temperature martensite phase remains after the stress has been removed and that the wires undergoes reverse transformation into the original phase when heated after the molding so that the composite can restore the original shape.
However, the reverse transformation finishing temperature (Af) of currently widely used heat-treated NITINOL (Ti-50at % Ni) is 100° C. or lower, whereas a heat-resisting epoxy resin is thermally cured at a temperature of 130° C. or higher. Therefore, when such TiNi wires are embedded in a matrix such as CFRP, GFRP or an epoxy resin and molded by curing, the curing temperature exceeds the reverse transformation finishing temperature thereof. Thus, unless the pre-strained wires are fixed during the course of the thermal curing and molding, the wires will shrink so that the shape memory effect of the TiNi wires will be no longer available thereafter.
Accordingly, the current practice requires the use of a jig to fix opposite ends of the TiNi wires in order to retain the pre-strain during the curing of the CFRP, GFRP or epoxy resin in which the TiNi wires are embedded. Thus, the functional composite material using shape memory alloy wires poses a great problem in practice, because the size and shape thereof are considerably limited.
Also, in the case where a pre-strain is given to a shape memory alloy by drawing, the yield stress in the martensite phase is so low that there is caused a problem that an effect to increase the strengths and rigidity at low temperatures is insufficient.
The present invention provides a functional composite material using a shape memory alloy and a method of preparing same which are devoid of the above-mentioned defects, in which the reverse transformation temperature of TiNi wires is increased to a temperature higher than a curing temperature for a matrix material such as CFRP, GFRP or an epoxy resin by cold working of the TiNi wires, which permit the TiNi wires to be embedded in the resin without fixing both ends thereof, and which do not cause reverse transformation or shrinkage of the TiNi wires during the curing.
The present invention also provides a method of preparing a functional composite material in which the reverse transformation temperature of the TiNi wires are returned to the normal state by heating the TiNi wires by passing an electric current therethrough for a short time, and which permits the utilization of shape memory effect of the TiNi wires.
The present invention further provides a functional composite material using a shape memory alloy and a method of preparing same in which only wire drawing in a wire production process is utilized in the cold working to generate a pre-strain therein and to increase the reverse transformation temperatures thereof, which can significantly reduce the manufacturing costs, and which can increase the yield stress in the martensite phase of the TiNi wires by the cold working and, therefore, can expect an effect of increasing the strengths and rigidity thereof.